In a radio communication system, radio signals typically travel from a transmitter to a receiver via a plurality of paths, each involving reflections from one or more scatterers. Received signals from the paths may interfere constructively or destructively at the receiver (resulting in position-dependent fading). Further, differing lengths of the paths, and hence the time taken for a signal to travel from the transmitter to the receiver, may cause inter-symbol interference.
It is well known that the above problems caused by multipath propagation can be mitigated by the use of multiple antennas at the receiver (receive diversity), which enables some or all of the multiple paths to be resolved. For effective diversity it is necessary that signals received by individual antennas have a low cross-correlation. Typically this is ensured by separating the antennas by a substantial fraction of a wavelength, although closely-spaced antennas may also be employed by using techniques disclosed in our International patent application WO01/71843 (applicant's reference PHGB000033). By ensuring use of substantially uncorrelated signals, the probability that destructive interference will occur at more than one of the antennas at any given time is minimised.
Similar improvements may also be achieved by the use of multiple antennas at the transmitter (transmit diversity). Diversity techniques may be generalised to the use of multiple antennas at both transmitter and receiver, known as a Multi-Input Multi-Output (MIMO) system, which can further increase system gain over a one-sided diversity arrangement. As a further development, the presence of multiple antennas enables spatial multiplexing, whereby a data stream for transmission is split into a plurality of sub-streams, each of which is sent via many different paths. One example of such a system is described in U.S. Pat. No. 6,067,290, another example, known as the BLAST system, is described in the paper “V-BLAST: an architecture for realising very high data rates over the rich-scattering wireless channel” by P W Wolniansky et al in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy, 29 Sep. to 2 Oct. 1998.
The performance gains which may be achieved from a MIMO system may be used to increase the total data rate at a given error rate, or to reduce the error rate for a given data rate, or some combination of the two. A MIMO system can also be controlled to reduce the total transmitted energy or power for a given data rate and error rate.
One area in which MIMO techniques may be applied is a High-Speed Downlink Packet Access (HSDPA) scheme, which is currently being developed for UMTS and which may facilitate transfer of packet data to a mobile station at up to 4 Mbps. In one proposed embodiment of HSDPA separate data streams are sent from respective antennas at a Base Station (BS), which data streams can in principle be received and decoded by a Mobile Station (MS) having at least as many antennas as there are data streams. An ARQ (Automatic Repeat reQuest) scheme is needed to ensure correct delivery of each data packet, since accurate data transmission is viewed as more important than the reduced system throughput under poor channel conditions (due to multiple re-transmissions).
A problem with the use of a MIMO system for packet data transmission is the impact of differing radio link qualities on the communication system. For example, some of the data streams may have very poor quality radio links, and if all the data is combined this will degrade the performance of the other links.